OK guys - after having a jolly good nap I tried again and I found out what I was doing wrong. You’re not gonna believe it lol:
It was the way I was turning the magneto key:
I am used to turn the key with the mousewheel. Normally I just roll-roll-roll and the plane starts.
Here I knew I have to hold it a bit longer so I was like: you gotta roll-roll-roll-rolll-roll ok?
No - you have to hold the LMB and it will start eventually.
Holy s**t - all this because of that? Well, I’m sorry to make you reply & participate so much (thanks for that!) but eventually we found out eh?
Happy flying I guess?
best
Merlin